U.S. patent number 5,585,921 [Application Number 08/404,660] was granted by the patent office on 1996-12-17 for laser-ultrasonic non-destructive, non-contacting inspection system.
This patent grant is currently assigned to Hughes Aircraft Company. Invention is credited to Gilmore J. Dunning, Marvin B. Klein, Phillip V. Mitchell, Thomas R. O'Meara, David M. Pepper.
United States Patent |
5,585,921 |
Pepper , et al. |
December 17, 1996 |
Laser-ultrasonic non-destructive, non-contacting inspection
system
Abstract
A laser-ultrasonic inspection system is provided for on-line and
off-line inspection of a workpiece. The system utilizes an optical
acoustic wave generation and detection system with relatively high
spatial resolution and which appreciably reduces the effects of
parasitic acoustic coupling. An array of acoustic waves are
generated in the workpiece by a short pulse optical transmitter bee
with a beam geometry that is tailored to focus the acoustic waves
at an inspection site in the workpiece. The acoustic waves that
probe the inspection site are then detected by reflecting an
optical read-out beam from a surface of the workpiece and optically
interfering it with an optical reference beam. The geometry of the
optical read-out beam is chosen such that the read-out beam only
detects the acoustic waves that arrive from the inspection site
(acoustic waves that arrive from other parasitic acoustic sources
are out of phase with respect to each other and cancel out). A
wavefront compensation system improves acoustic clutter rejection
and also improves the signal-to-noise by compensating for phase and
amplitude aberrations induced on the optical read-out beam by the
optically rough surface of the workpiece.
Inventors: |
Pepper; David M. (Malibu,
CA), O'Meara; Thomas R. (Malibu, CA), Mitchell; Phillip
V. (Simi Valley, CA), Dunning; Gilmore J. (Newbury Park,
CA), Klein; Marvin B. (Pacific Palisades, CA) |
Assignee: |
Hughes Aircraft Company (Los
Angeles, CA)
|
Family
ID: |
23600515 |
Appl.
No.: |
08/404,660 |
Filed: |
March 15, 1995 |
Current U.S.
Class: |
356/487; 356/432;
356/502 |
Current CPC
Class: |
G01N
29/075 (20130101); G01N 29/2418 (20130101); G01N
2291/0422 (20130101); G01N 2291/0423 (20130101); G01N
2291/0427 (20130101) |
Current International
Class: |
G01N
29/04 (20060101); G01N 29/24 (20060101); G01N
29/07 (20060101); G01B 009/02 () |
Field of
Search: |
;356/349,351,352,357,358,432T ;73/655,657 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Matsuda et al., "Optical Detection of Transient Lamb Waves on Rough
Surfaces by a Phase-Conjugate Method", Japanese Journal of Applied
Physics, vol. 31, pp. 987-989 (1992). .
Ing et al., "Broadband Optical Detection of Ultrasound by Two-Wave
Mixing in a Photorefractive Crystal", Applied Physics Letters, vol.
59, pp. 3233-3235 (1991). .
Paul et al., "Interferometric Detection of Ultrasound at Rough
Surfaces Using Optical Phase Conjugation", Applied Physics Letters,
vol. 50, pp. 1569-1571 (1987). .
Blouin, "Detection of Ultrasonic Motion of a Scattering Surface by
Two-Wave Mixing in a Photorefractive GaAs Crystal", Applied Physics
Letters, vol. 65, pp. 3932-3934 (1994). .
Monchalin, "Optical Detection of Ultrasound at a Distance Using a
Confocal Fabry-Perot Interferometer", Applied Physics Letters, vol.
47, pp. 14-16 (1985). .
Celestino J. Gaeta et al., "Characteristics of Innovative
Adaptive-Optics Servos That Use Membrane-Based Spatial Light
Modulators", J. Opt. Soc. Am. A., vol. 11, No. 2, pp. 880-894
(1994). .
Delaye, et al., "Heterodyne Detection of Ultrasound from Rough
Surfaces Using a Double Phase Conjugate Mirror", Applied Physics
Letters, vol. 67, pp. 3251-3253 (1995). .
Nakano et al., "Optical Detection of Ultrasound on Rough Surfaces
by a Phase-Conjugate Method", Ultrasonics, vol. 33, No. 4, pp.
261-264 (1995). .
C. B. Scruby and L. E. Drain, Laser Ultrasonics, Techniques and
Applications, Adam Hilger, New York (1990), pp. 325-350..
|
Primary Examiner: Turner; Samuel A.
Attorney, Agent or Firm: Duraiswamy; V. D. Denson-Low; W.
K.
Claims
We claim:
1. A laser-ultrasonic, non-contacting inspection system,
comprising:
an acoustic wave generator for generating an array of acoustic
waves in a workpiece that arrive substantially in phase with
respect to each other at a desired inspection area in said
workpiece and propagate through and/or reflect from said inspection
area arrive at an area on an outer surface of said workpiece, with
said waves arriving substantially in phase with respect to each
other over a locus at said outer surface, thereby vibrating said
outer surface over said locus,
an acoustic wave receiver that detects said acoustic waves over
said locus, said acoustic wave receiver comprising an optical beam
generator that directs an optical read-out beam to said vibrating
surface and distributes it over said locus so that the read-out
beam is phase modulated by the vibrations along said locus and
reflected from said surface, and
a signal monitor for extracting information about said inspection
area from said acoustic wave receiver.
2. The system of claim 1, wherein said workpiece comprises first
and second workpieces, said inspection area is located between said
first and second workpieces, and said acoustic wave generator
comprises an optical beam generator for generating an optical beam
having a power density sufficient to generate an array of acoustic
waves in said first workpiece through either thermoelastic or
ablation effects.
3. The system of claim 1, wherein said acoustic wave generator
comprises an optical beam generator for directing an optical beam
that has a power density sufficient to generate an array of
acoustic waves in said workpiece through either thermoelastic or
ablation effects onto said workpiece so that said beam is symmetric
about said inspection area.
4. The system of claims 3, wherein said symmetric beam falls within
a ring-shaped pattern on said first workpiece.
5. The system of claim 4, wherein said beam illuminates a half to
full ring-shaped pattern on said first workpiece.
6. The system of claim 3, wherein said optical beam generator
distributes said read-out beam over said locus so that it is
symmetric about said inspection area.
7. The system claim 6, wherein said symmetric read-out beam falls
within a ring-shaped pattern on said vibrating surface.
8. The system of claim 7, wherein said read-out beam illuminates a
half to full ring-shaped pattern on said workpiece.
9. A method of inspecting an area in a workpiece, said workpiece
also including an outer surface, comprising the steps of:
generating an array of acoustic waves in said workpiece that arrive
substantially in phase with respect to each other at said
inspection area and propagate through and/or reflect from said
inspection area to arrive at an area on an outer surface of said
workpiece, with said waves arriving substantially in phase with
respect to each other over a locus at said outer surface, thereby
vibrating said outer surface over said locus,
detecting said acoustic waves over said locus by directing an
optical read-out beam to said vibrating surface and distributing it
over said locus so that said read-out beam is phase modulated by
said vibrations and reflected from said surface, and
extracting information about said inspection area from said
detected acoustic waves.
10. The method of claim 9, wherein said workpiece comprises first
and second workpieces, said inspection area is located between said
first and second workpieces, and said array of acoustic waves is
generated in said first workpiece by directing an optical beam
having a power density sufficient to generate said array of
acoustic waves through thermoelastic or ablation effects onto said
first workpiece.
11. The method of claim 9, wherein said array of acoustic waves is
generated in said workpiece by directing an optical beam that has a
power density sufficient to generate an array of acoustic waves in
said workpiece through either thermoelastic or ablation effects
onto said workpiece so that said beam is symmetric about said
inspection area.
12. The method of claim 11, wherein said optical beam is directed
to fall within a ring-shaped pattern on said first workpiece.
13. The method of claim 12, wherein said optical beam illuminates a
half to full ring-shaped pattern on said workpiece.
14. The method of claim 9, wherein said optical read-out beam is
distributed over said locus so that it is symmetric about said
inspection area.
15. The method of claim 14, wherein said read-out beam is directed
to fall within a ring-shaped pattern on said vibrating surface.
16. The method of claim 15, wherein said read-out beam illuminates
a half to full ring-shaped pattern on said vibrating surface.
17. The method of claim 9, further comprising the step of removing
phase and amplitude aberrations induced on said optical read-out
beam.
18. The method of claim 17, wherein said phase and amplitude
aberrations are removed with a double-pumped phase conjugate
mirror.
19. The method of claim 17, wherein said phase and amplitude
aberrations are removed with a deformable mirror.
20. The method of claim 17, wherein said aberrations are removed
with a membrane-based spatial light modulator.
21. The method of claim 9, wherein information about said
inspection area is extracted from said acoustic waves by:
generating an optical reference beam having a predetermined phase
relationship with said read-out beam,
combining said reference beam with said phase modulated read-out
beam to generate a beat frequency between said beams, and
extracting information about said inspection area from the beat
frequency between said reference beam and said modulated read-out
beam.
22. A weld inspection system for on-line inspection of a weld site
between first and second workpieces, each of said workpieces
including an outer surface, comprising:
an acoustic wave generator for generating an array of acoustic
waves in said first workpiece that arrive substantially in phase
with respect to each other at said weld site such that a portion of
said waves reflect from said weld site and propagate back through
said first workpiece to arrive at and vibrate an area on the outer
surface of said first workpiece, with said waves arriving
substantially in phase with respect to each other over a locus at
said outer surface, and another portion of said acoustic waves
propagate into said second piece through said weld site as a weld
joint forms,
an acoustic wave receiver that detects said acoustic waves over
said locus by directing an optical read-out beam to said vibrating
surface and distributing it over said locus so that the read-out
beam is phase modulated by the vibrations along said locus and
reflected from said surface, and
a signal monitor for extracting information about said weld site
from said acoustic wave receivers.
23. The system of claim 22, wherein said acoustic wave generator
comprises:
a pulsed laser for generating a pulsed transmitter beam with a
power density sufficient to generate an array of acoustic waves in
said first workpiece through thermoelastic or ablation effects,
and
a transmitter beam director for directing said transmitter beam to
said outer surface of said first workpiece.
24. The system of claim 23, wherein said transmitter beam director
comprises:
a transmitter beam expander for expanding the diameter of said
transmitter beam,
a transmitter beam reflector for directing said transmitter beam to
said first workpiece, and
a transmitter beam focuser for focusing said transmitter beam onto
said first workpiece.
25. The system of claim 24, wherein said transmitter beam reflector
comprises an annular mirror for reflecting said transmitter beam
while simultaneously allowing a weld beam to access said first and
second workpieces.
26. The system of claim 25, wherein said transmitter beam focuser
comprises:
an annular lens, and
a dispersive annular axicon,
said annular lens and axicon forming said transmitter beam into a
ring-shaped light pattern on said outer surface of said first
workpiece, said annular lens and axicon simultaneously allowing
said weld beam to access said workpieces while said transmitter and
read-out beams are directed to said first workpiece.
27. The system of claim 22, wherein said acoustic wave receiver
comprises:
an optical beam generator for generating said optical read-out beam
and a reference beam, and
a read-out beam director for directing said read-out beam to said
vibrating surface of said first workpiece such that said read-out
beam is reflected from said vibrating surface and phase modulated
by vibrations induced on by said acoustic waves.
28. The system of claim 27, wherein said read-out beam director
comprises:
a read-out beam expander for expanding the diameter of said
read-out beam,
a read-out beam reflector for directing said read-out beam to said
first workpiece, and
a read-out beam focuser for focusing said read-out beam onto said
first workpiece.
29. The system of claim 28, wherein said read-out beam reflector
comprises an annular mirror for reflecting said read-out beam while
simultaneously allowing a weld beam to access said workpieces.
30. The system of claim 29, wherein said read-out beam focuser
comprises:
an annular lens, and
a dispersive annular axicon,
said annular lens and axicon forming a ring-shaped light pattern at
said outer surface over said locus, said annular lens and axicon
simultaneously allowing said weld beam to access said workpieces
while said transmitter and read-out beams are directed to said
first workpiece.
31. The system of claim 30, wherein said transmitter and read-out
beams have different wavelengths.
32. The system of claim 31, wherein said transmitter and read-out
beam directors are implemented with common elements.
33. The system of claim 27, wherein said signal monitor
comprises:
an optical beam generator for generating an optical reference beam
having a predetermined phase relationship with said read-out
beam,
a beam combiner for combining said reference beam and said phase
modulated read-out beam to generate a beat frequency between said
beams,
a detector for detecting the beat frequency between said reference
beam and said phase modulated read-out beam, and
a processor for extracting information about said weld joint from
said beat frequency.
34. The system of claim 27, further comprising a wavefront
compensator for removing phase and amplitude aberrations induced in
said phase modulated read-out beam, thereby wavefront compensating
said modulated read-out beam.
35. The system of claim 34, wherein said signal monitor
comprises:
a beam combiner for combining said wavefront compensated modulated
read-out beam and said reference beam,
a detector for detecting the beat frequency between said reference
beam and said wavefront compensated modulated read-out beam,
and
a processor for extracting information about said weld joint from
said beat frequency.
36. The system of claim 34, wherein said wavefront compensator
comprises:
a beam divider for dividing said reference beam into a compensator
reference beam and a detector reference beam,
a membrane-based spatial light modulator having a readout side for
receiving said modulated read-out beam, a write side for receiving
said compensator reference beam, and micro-mirrors on said readout
side for reflecting said modulated read-out beam, said spatial
light modulator removing said phase and amplitude aberrations from
said modulated read-out beam, thereby wavefront compensating said
modulated read-out beam, and
a first beam director for directing said reflected, modulated
read-out beam to said write side, thereby forming a feedback
loop,
said reflected, modulated read-out beam optically interfering with
said compensator reference beam at said write side to create an
optical interference pattern having an intensity profile on said
write side that corresponds to said phase and amplitude aberrations
imparted on said modulated read-out beam,
said interference pattern on said write side causing a
corresponding change in the position of said micro-mirrors on said
readout side to correct said phase and amplitude aberrations as
said modulated read-out beam undergoes reflection from said
micro-mirrors.
37. The system of claim 36, wherein said signal monitor
comprises:
a beam combiner for combining said wavefront compensated modulated
read-out beam and said detector reference beam,
a detector for detecting the beat frequency between said detector
reference beam and said wavefront compensated modulated read-out
beam, and
a processor for extracting information about said weld joint from
said beat frequency.
38. The system of claim 35, wherein said wavefront compensator
comprises:
a deformable mirror for receiving and reflecting said modulated
read-out beam,
an array of phase detectors for receiving said reflected, modulated
read-out beam, each of said phase detectors in electrical
communication with a corresponding area of said deformable mirror,
and
a first beam director for directing said reflected, modulated
read-out beam to said array of detectors for detecting said phase
and amplitude aberrations over an incremental portion of said
read-out beam and causing a corresponding change in the position of
said corresponding area of said deformable mirror to correct said
phase and amplitude aberrations as said read-out beam reflects from
said deformable mirror.
39. The system of claim 34, wherein said wavefront compensator
comprises a double-pumped phase conjugate mirror system.
40. The system of claim 39, wherein said double-pumped phase
conjugate mirror system comprises:
a first beam divider for dividing said reference beam into a
compensator reference beam and a detector reference beam, and
a photorefractive crystal for receiving said modulated read-out
beam and said compensator reference beam, said modulated read-out
beam and said compensator reference beam writing photorefractive
diffraction gratings in said crystal that diffract said modulated
read-out beam and removes said phase and amplitude aberrations from
said modulated read-out beam, thereby wavefront compensating said
modulated read-out beam.
41. The system of claim 40, wherein said signal monitor
comprises:
a beam combiner for combining said wavefront compensated modulated
read-out beam and said detector reference beam,
a detector for detecting the beat frequency between said detector
reference beam and said wavefront compensated modulated read-out
beam, and
a processor for extracting information about said weld joint from
said beat frequency.
42. The system of claim 40, further comprising a frequency shifter
for shifting the frequency of said compensator reference beam to
increase the efficiency of said double-pumped phase conjugator.
43. A weld inspection system for off-line inspection of a weld
joint between first and second workpieces, each of said workpieces
also including an outer surface, comprising:
an acoustic wave generator for generating an array of acoustic
waves in said first workpiece that arrive substantially in phase
with respect to each other at said weld joint such that a portion
of said waves propagate through said weld joint to arrive at and
vibrate an area on the outer surface of said second workpiece, with
said waves arriving substantially in phase with respect to each
other over a locus at said outer surface,
an acoustic wave receiver that detects said acoustic waves over
said locus by directing an optical read-out beam to said vibrating
surface and distributing it over said locus so that the read-out
beam is phase modulated by the vibrations along said locus and
reflected from said surface, and
a signal monitor for extracting information about said weld site
from said acoustic wave receivers.
44. The system of claim 43, wherein said acoustic wave generator
comprises:
a pulsed laser for generating a pulsed transmitter beam with a
power density sufficient to generate an array of acoustic waves in
said first workpiece through thermoelastic or ablation effects,
and
a transmitter beam director for directing said transmitter beam to
said outer surface of said first workpiece.
45. The system of claim 44, wherein said transmitter beam director
comprises:
a transmitter beam expander for expanding the diameter of said
transmitter beam,
a transmitter beam reflector for directing said transmitter beam to
said first workpiece, and
a transmitter beam focuser for focusing said transmitter beam on to
said first workpiece.
46. The system of claim 45, wherein said transmitter beam focuser
comprises:
a lens, and
a dispersive axicon,
said lens and axicon forming said transmitter beam into a
ring-shaped light pattern on said outer surface of said first
workpiece.
47. The system of claim 43, wherein said acoustic wave receiver
comprises:
an optical beam generator for generating said optical read-out beam
and a reference beam, and
a read-out beam director for directing said read-out beam to said
vibrating surface of said second workpiece such that said read-out
beam is reflected from said vibrating surface and phase modulated
by vibrations induced by said acoustic waves.
48. The system of claim 47, wherein said read-out beam director
comprises:
a read-out beam expander for expanding the diameter of said
read-out beam,
a read-out beam reflector for directing said read-out beam to said
first workpiece, and
a read-out beam focuser for focusing said read-out beam onto said
first workpiece.
49. The system of claim 48, wherein said read-out beam focuser
comprises:
a lens, and
a dispersive axicon,
said lens and axicon forming a ring-shaped light pattern at said
outer surface over said locus.
50. The system of claim 47, wherein said signal monitor
comprises:
a beam combiner for combining said reference beam and said phase
modulated read-out beam,
a detector for detecting the beat frequency between said reference
beam and said phase modulated read-out beam, and
a processor for extracting information about said weld joint from
said beat frequency.
51. The system of claim 47, further comprising a wavefront
compensator for removing phase and amplitude aberrations induced in
said phase modulated read-out beam, thereby wavefront compensating
said modulated read-out beam.
52. The system of claim 51, wherein said signal monitor
comprises:
an optical beam generator for generating an optical reference beam
having a predetermined phase relationship with said read-out
beam,
a beam combiner for combining said reference beam and said phase
modulated read-out beam to generate a beat frequency between said
beams,
a detector for detecting the beat frequency between said reference
beam and said phase modulated readout beam, and
a processor for extracting information about said weld joint from
said beat frequency.
53. The system of claim 51, wherein said wavefront compensator
comprises:
a beam divider for dividing said reference beam into a compensator
reference beam and a detector reference beam,
a membrane-based spatial light modulator having a readout side for
receiving said modulated read-out beam, a write side for receiving
said compensator reference beam, and micro-mirrors on said readout
side for reflecting said modulated read-out beam, said spatial
light modulator removing said phase and amplitude aberrations from
said modulated read-out beam, thereby wavefront compensating said
modulated read-out beam, and
a first beam director for directing said reflected, modulated
read-out beam to said write side, thereby forming a feedback
loop,
said reflected, modulated read-out beam optically interfering with
said compensator reference beam at said write side to create an
optical interference pattern having an intensity profile on said
write side that corresponds to said phase and amplitude aberrations
imparted on said modulated read-out beam,
said interference pattern on said write side causing a
corresponding change in the position of said micro-mirrors on said
readout side to correct said phase and amplitude aberrations as
said modulated read-out beam undergoes reflection from said
micro-mirrors.
54. The system of claim 53, wherein said signal monitor
comprises:
a beam combiner for combining said wavefront compensated modulated
read-out beam and said detector reference beam,
a detector for detecting the beat frequency between said detector
reference beam and said wavefront compensated modulated read-out
beam, and
a processor for extracting information about said weld joint from
said beat frequency.
55. The system of claim 52, wherein said wavefront compensator
comprises:
a deformable mirror for receiving and reflecting said modulated
read-out beam,
an array of phase detectors for receiving said reflected, modulated
read-out beam, each of said phase detectors in electrical
communication with a corresponding area of said deformable mirror,
and
a first beam director for directing said reflected, modulated
read-out beam to said array of detectors for detecting said phase
and amplitude aberrations over an incremental portion of said
read-out beam and causing a corresponding change in the position of
said corresponding area of said deformable mirror to correct said
phase and amplitude aberrations as said read-out beam reflects from
said deformable mirror.
56. The system of claim 51, wherein said wavefront compensator
comprises a double-pumped phase conjugate mirror system.
57. The system of claim 56, wherein said double-pumped phase
conjugate mirror system comprises:
a first beam divider for dividing said reference beam into a
compensator reference beam and a detector reference beam, and
a photorefractive crystal for receiving said modulated read-out
beam and said compensator reference beam, said modulated read-out
beam and said compensator reference beam writing photorefractive
diffraction gratings in said crystal that diffract said modulated
read-out beam and removes said phase and amplitude aberrations from
said modulated read-out beam, thereby wavefront compensating said
modulated read-out beam.
58. The system of claim 57, wherein said signal monitor
comprises:
a beam combiner for combining said wavefront compensated modulated
read-out beam and said detector reference beam,
a detector for detecting the beat frequency between said detector
reference beam and said wavefront compensated modulated read-out
beam, and
a processor for extracting information about said weld joint from
said beat frequency.
59. A laser-ultrasonic, non-contacting inspection system,
comprising:
an acoustic wave generator for generating an array of acoustic
waves in a workpiece that arrive substantially in phase with
respect to each other at a desired inspection area in said
workpiece and propagate through and/or reflect from said inspection
area to illuminate an area on an outer surface of said workpiece,
with said waves arriving substantially in phase with respect to
each other over a locus at said outer surface, thereby vibrating
the illuminated surface, said generator comprising an optical beam
generator that directs an optical beam that has a power density
sufficient to generate an array of acoustic waves in said workpiece
through either thermoelastic or ablation effects onto said
workpiece so that said beam is symmetric about said inspection
area,
an acoustic wave receiver that detects said acoustic waves over
said locus, said acoustic wave receiver comprising an optical beam
generator that directs a read-out beam onto said vibrating surface
so that said read-out beam falls within a ring-shaped pattern on
said vibrating surface that is symmetric about said inspection
area, said read-out beam being phase modulated by said vibrations
and reflected from said surface, and
a signal monitor for extracting information about said inspection
area from said acoustic wave receiver.
60. The system of claim 59, wherein said read-out beam illuminates
a half to full ring-shaped pattern on said workpiece.
61. A method of inspecting an area in a workpiece, said workpiece
also including an outer surface, comprising the steps of:
generating an array of acoustic waves in said workpiece that arrive
substantially in phase with respect to each other at said
inspection area and propagate through and/or reflect from said
inspection area to illuminate an area on an outer surface of said
workpiece, with said waves arriving substantially in phase with
respect to each other over a locus at said outer surface, thereby
vibrating the illuminated surface, said acoustic waves being
generated in said workpiece by directing an optical beam that has a
power density sufficient to generate an array of acoustic waves in
said workpiece through either thermoelastic or ablation effects
onto said first workpiece so that said beam is symmetric about said
inspection area,
detecting said acoustic waves over said locus by directing a
read-out beam onto said vibrating surface so that said read-out
beam falls within a ring-shaped pattern on said vibrating surface
that is symmetric about said inspection area, is phase modulated by
said vibrations and is reflected from said surface, said read-out
beam illuminating a half to full ring-shaped pattern on said
vibrating surface, and
extracting information about said inspection area from said
detected acoustic waves.
62. A method of inspecting an area in a workpiece, said workpiece
also including an outer surface, comprising the steps of:
generating an array of acoustic waves in said workpiece that arrive
substantially in phase with respect to each other at said
inspection area and propagate through and/or reflect from said
inspection area to arrive at an area on an outer surface of said
workpiece, with said waves arriving substantially in phase with
respect to each other over a locus at said outer surface, thereby
vibrating said outer surface over said locus,
detecting said acoustic waves over said locus by directing an
optical read-out beam onto said vibrating surface at said locus so
that the read-out beam is phase modulated by said vibrations and
reflected from said surface,
removing phase and amplitude aberrations induced on said optical
read-out beam,
extracting information about said inspection area from said
detected acoustic waves.
63. A weld inspection system for on-line inspection of a weld site
between first and second workpieces, each of said workpieces
including an outer surface, comprising:
a pulsed laser that generates a pulsed transmitter beam,
a transmitter beam expander that expands the diameter of said
transmitter beam,
a transmitter beam reflector that directs said transmitter beam to
the outer surface of said first workpiece, said reflector
comprising an annular mirror that reflects said transmitter beam
while simultaneously allowing a weld beam to access said first and
second workpieces,
a transmitter beam focuser that focuses said transmitter beam onto
the outer surface of said first workpiece, said transmitter beam
having a power density sufficient to generate an array of acoustic
waves in said first workpiece through thermoelastic or ablation
effects, said acoustic waves arriving substantially in phase with
respect to each other at said weld site such that a portion of said
waves reflect from said weld site and propagate back through said
first workpiece to illuminate and vibrate an area on the outer
surface of said first workpiece, with said waves arriving
substantially in phase with respect to each other over a locus at
said outer surface, and another portion of said acoustic waves
propagate into said second piece through said weld site as a weld
joint forms,
an acoustic wave receiver that detects said acoustic waves over
said locus, and
a signal monitor for extracting information about said weld site
from said acoustic wave receivers.
64. The system of claim 63, wherein said transmitter beam focuser
comprises:
an annular lens, and
a dispersive annular axicon,
said annular lens and axicon forming said transmitter beam into a
ring-shaped light pattern on said outer surface of said first
workpiece, said annular lens and axicon simultaneously allowing
said weld beam to access said workpieces while said transmitter and
read-out beams are directed to said first workpiece.
65. A weld inspection system for on-line inspection of a weld site
between first and second workpieces, each of said workpieces
including an outer surface, comprising:
an acoustic wave generator for generating an array of acoustic
waves in said first workpiece that arrive substantially in phase
with respect to each other at said weld site such that a portion of
said waves reflect from said weld site and propagate back through
said first workpiece to illuminate and vibrate an area on the outer
surface of said first workpiece, with said waves arriving
substantially in phase with respect to each other over a locus at
said outer surface, and another portion of said acoustic waves
propagate into said second piece through said weld site as a weld
joint forms,
an optical beam generator that generates an optical read-out beam
and reference beam,
a read-out beam expander that expands the diameter of said read-out
beam,
a read-out beam reflector that directs said read-out beam to said
first workpiece such that said read-out beam is reflected from said
vibrating surface and phase modulated by vibrations induced by said
acoustic waves, said read-out beam reflector comprising an annular
mirror that reflects said read-out beam while simultaneously
allowing a weld beam to access said workpieces,
a read-out beam focuser that focuses said read-out beam onto said
first workpiece, and
a signal monitor for extracting information about said weld site
from said phase modulated read-out beam.
66. The system of claim 65, wherein said read-out beam focuser
comprises:
an annular lens, and
a dispersive annular axicon,
said annular lens and axicon forming a ring-shaped light pattern at
said outer surface over said locus, said annular lens and axicon
simultaneously allowing said weld beam to access said workpieces
while said transmitter and read-out beams are directed to said
first workpiece.
67. The system of claim 66, wherein said transmitter and read-out
beams have different wavelengths.
68. The system of claim 31, wherein said transmitter and read-out
beam directors are implemented with common elements.
69. A weld inspection system for off-line inspection of a weld
joint between first and second workpieces, each of said workpieces
also including an outer surface, comprising:
an acoustic wave generator for generating an array of acoustic
waves in said first workpiece that arrive substantially in phase
with respect to each other at said weld joint such that a portion
of said waves propagate through said weld joint to illuminate and
vibrate an area on the outer surface of said second workpiece, with
said waves arriving substantially in phase with respect to each
other over a locus at said outer surface,
an optical beam generator that generates an optical read-out beam
and reference beam,
a read-out beam expander that expands the diameter of said read-out
beam,
a read-out beam reflector that directs said read-out beam to said
first workpiece such that said read-out beam is reflected from said
vibrating surface and phase modulated by vibrations induced by said
acoustic waves,
a read-out beam focuser that focuses said read-out beam onto said
first workpiece, said focuser comprising a lens and dispersive
axicon that together form a ring-shaped light pattern at said
vibrating surface over said locus, and
a signal monitor for extracting information about said weld site
from said acoustic wave receivers.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to weld inspection systems and more
specifically to an efficient, non-contacting, laser-ultrasonic weld
inspection system for on-line and off-line use.
2. Description Of the Related Art
Automated welding systems are commonly used in many industrial
applications. During the welding process, various factors can
introduce structural flaws and defects in the weld, causing a
reduction in the fusion width, and hence the strength and
reliability of the welded part.
These defects are often hidden and impossible to observe visually.
For example, the fusion width of the weld band may be of an
incorrect size, the defect could be a small crack in the weld, or
the weld itself may be located between layers of opaque material.
In addition, typical automated weld systems operate at high
throughput speeds. Therefore, a weld defect caused by a malfunction
in the welding system could be reproduced many times before the
problem is discovered.
Consequently, there is a need for reliable and accurate weld
inspections systems that are fast enough to keep up with automated
welding systems. Ideally, the weld inspection system should be
capable of on-line operation to diagnose faulty welds early in the
welding process. In addition, the on-line system should be capable
of use as a transducer in a closed-loop welder control system, so
that each weld is optimally formed and is certified to a given
specification. The system should also have high spatial resolution
in order to detect very small defects in the weld that would
otherwise go undetected.
There are several off-line and on-line techniques available for
weld inspection. One off-line technique involves taking sample
parts from the assembly process on a periodic basis and analyzing
the welds. Frequently, the analysis involves breaking or sawing
through the weld itself to determine its strength and/or size.
However, this technique destroys potentially useful parts in the
process. In addition, since only the samples are analyzed, many
untested welds or weld areas (which may exist immediately adjacent
to the sectioned region) are allowed to pass through untested,
resulting in a statistically unreliable evaluation procedure.
Some of the prior on-line techniques, such as that disclosed in
U.S. Pat. No. 5,121,339, entitled "LASER WELD FAULT DETECTION
SYSTEM", issued Jun. 9, 1992 to D. Jenuwine, et. al., analyze the
weld during the welding process by sampling and analyzing high
frequency emissions that emanate from the weld itself. The high
frequency emissions are sampled either directly by using ultrasonic
sensors that are placed in physical contact with the part being
welded, or indirectly by non-contacting sensors that sense and
analyze the airborne emissions from the weld.
The contacting acoustic emission detection systems are expensive,
unreliable, and difficult to set up and calibrate. The
non-contacting emission detection systems are difficult to
calibrate, especially if weld conditions are not identical from
part to part. In addition, the non-contacting systems are very
sensitive to ambient noise, which can reduce the accuracy of the
weld analysis.
Neither of these emission detection systems are very effective in
giving detailed information about the weld. They are more useful
for qualitative pass/fail determinations rather than quantitative,
high resolution information about the weld.
Furthermore, existing contacting and close proximity systems can
not be readily implemented in the adverse environments that are
typically found in the case of in-factory operation. These
environments may include high temperature, plasma, vacuum, or
radiation environments. In addition, existing systems are not
robust, because they must be "matched" to the surface contours of a
given workpiece.
Laser-ultrasonic techniques have been used in both on-line and
off-line systems. For some examples of laser-ultrasonic flaw
detection techniques, see C. B. Scruby and L. E. Drain, Laser
Ultrasonics, Techniques and Applications, Adam Hilger, New York
(1990), pages 325-350. The technique typically used by prior
laser-ultrasonic systems is illustrated in FIGS. 1aand 1b.
A pulsed transmitter laser beam 20 is focused to a single spot 22
on the outer surface 24 of a workpiece 26, typically made of metal,
with an internal crack or flaw 32. The transmitter beam 20
generates acoustic waves 28 in the first workpiece 26 which
propagate over a broad angular range, so that a portion of the
waves 28 illuminate the flaw 32. The acoustic waves 28 are
reflected by the flaw 32 to a second workpiece surface 30, and
cause the surface 30 to vibrate. A read-out laser beam 36 is
focused to a spot 38 on the second workpiece surface 30. The
vibrations induced by the reflected acoustic waves 31 phase
modulate the read-out beam 36, which is reflected by the second
surface 30. The reflected, modulated read-out beam is then
optically interfered with a reference beam (not shown) or is
directed to a frequency discriminator, such as a Fabry Perot
cavity, and the resulting interference pattern is analyzed by
receiver electronics to extract information about the existence and
size of the flaw 32.
Since the amount of acoustic energy that is reflected to the second
workpiece surface 30 increases as the size of the flaw 32
increases, this technique can be used to determine both the
existence and the size of the flaw 32, as illustrated by the
example of a signal vs. time plot of FIG. 1b.
One of the problems with this technique is illustrated in FIGS. 1c
and 1d. In many cases, parasitic acoustic coupling paths are
present in the system. These parasitic paths could be formed by
reflection of acoustic waves from an edge 39 of the workpiece 26,
or from other flaws. The parasitic acoustic waves 42 that are
reflected from the edge 39 can interfere with or even dominate the
acoustic signal detected by the read-out beam 36. This could
greatly disturb and, in some instances, completely void the
measurement process as illustrated in the signal vs. time plot of
FIG. 1d.
Another problem with prior laser-ultrasonic systems that utilize
interferometric laser receivers is that the efficiency of the
coherent detection required for interferometric measurements is
greatly reduced when the read-out beam is reflected from the rough
surface of the welded workpiece. Since the reflected read-out beam
is being coherently combined with a reference beam, the read-out
beam must have temporal and spatial coherence relative to the
reference beam. The reflection from the rough surface produces a
"speckle" field distribution on the optical detector that is used
to detect the interference pattern. The spatial coherence of the
reflected read-out beam is only maintained over a single "speckle"
width. If the phase aberrations on the reflected read-out beam are
not corrected (or compensated), only a small part or, equivalently,
a single spatial mode of the reflected read-out beam will be able
to coherently combine with the reference beam (only the area of the
reflected read-out beam over which spatial coherence is
maintained). The resulting detector signal is thousands of times
weaker than it would have been if the surface of the welded
workpiece had been a perfect mirror surface.
There are several schemes which employ some form of robust laser
ultrasonic receiver to sense minute vibrations at high bandwidths
in the presence of rough-cut workpiece surfaces:
(1) Phase-Conjugate Compensation Scheme
Systems that utilize phase-conjugate compensation schemes, such as
those described in Paul et al., "Interferometric detection of
ultrasound at rough surfaces using optical phase conjugation",
Applied Physics Letters, Vol. 50, pages 1569-1571 (1987), and
Matsuda et al., "Optical Detection of Transient Lamb Waves on Rough
Surfaces by a Phase-Conjugate Method", Japanese Journal of Applied
Physics, Vol. 31, pages 987-989 (1992), utilize a double-pass
optical architecture in which a laser probe beam illuminates the
workpiece surface under inspection. The probe beam is modified
temporally (by the desired ultrasound) and spatially (by the rough
workpiece surface). The probe beam portion that is scattered and/or
reflected by the workpiece surface is directed onto a
phase-conjugate mirror. The conjugate wave (wavefront-reversed
replica of the scattered and/or reflected probe beam) then retraces
its path back to the workpiece surface and, after reflection from
the surface, has its spatial wavefront restored back to its initial
(planar) wavefront. However, the conjugate wave (return beam) is
now "doubly" encoded with the desired ultrasound information as a
result of the two reflections from the workpiece surface. The fact
that the return beam is now planar enables one to more efficiently
detect the ultrasound via coherent detection techniques.
There are drawbacks to this approach. First, unless all the
scattered light is collected, the return beam's wavefront will not
be perfectly restored to its original planar shape. Second, the
beam must be reflected twice off the rough workpiece surface. If
the workpiece surface has low reflectivity, the detected optical
power will be greatly reduced. If there are local reflectivity
"drop-outs" on the workpiece surface (due to scratches, digs, rust
spots, blemishes, etc.), the spatial amplitude drop-outs will be
impressed onto the double-reflected return beam, resulting in a
reduction in the sensitivity of the system.
(2) Two-Wave Mixing Schemes
In systems that utilize a two-wave mixing scheme, such as the ones
described in Ing et al., "Broadband optical detection of ultrasound
by two-wave mixing in a photorefractive crystal", Applied Physics
Letters, Vol. 59, pages 3233-3235 (1991), and Blouin et al.,
"Detection of ultrasonic motion of a scattering surface by two-wave
mixing in a photorefractive GaAs crystal", Applied Physics Letters,
Vol. 65, pages 932-934 (1994), only a single pass off the workpiece
surface is required. The reflected signal beam is combined in a
photorefractive crystal with a planar "pump" beam. Energy from the
planar pump beam is diverted in the direction of the aberrated
signal beam. The pump beam forms the local oscillator, so that
coherent detection (either homodyne or heterodyne) can be
performed. These schemes suffer from several problems. First, the
pump beam must be coherent with respect to the signal beam over a
period of time equal to the response time of the photorefractive
crystal. Coherence is required to form the photorefractive gratings
required for energy exchange between the beams.
Second, if the reflected signal beam going into the photorefractive
crystal is highly speckled, the resultant output beam will likewise
be speckled, causing amplitude fluctuations that reduce the
signal-to-noise ratio by a factor of approximately 2. In addition,
the intensity fluctuations that also arise from speckle may cause
local depletion effects that adversely affect the wavefront
matching between the signal and diverted pump waves. This will
create a spatial mismatch between the signal and pump waves, which
will result in reduced coherent detection sensitivity.
(3) Fabry Perot Cavity Scheme
Systems that utilize this scheme, such as the one described in
Monchalin, "Optical detection of ultrasound at a distance using a
confocal Fabry-Perot interferometer" Applied Physics Letters, Vol.
47, pages 14-16 (1985), utilize a Fabry Perot interferometer, which
is basically a time-delayed, self-referencing interferometer (also
called a "discriminator"), whose output is proportional to the
velocity of the surface under inspection. In contrast, the two
schemes described above measure the displacement of the workpiece
surface. The Fabry Perot scheme has many drawbacks. First, the
output response is only linearly proportional to the workpiece
surface velocity over a finite bandwidth. Second, the field-of-view
is relatively small. Third, the Fabry Perot cavity length must be
long enough so that its free spectral range is compatible with the
bandwidth of the signal to be detected. This results in relatively
long devices (typically longer than one foot). Fourth, servo
controls are needed to properly stabilize the Fabry Perot cavity
length to the correct operating (bias) point.
SUMMARY OF THE INVENTION
In view of the above problems, the present invention provides a
laser-ultrasonic weld inspection system that has relatively high
spatial resolution, that drastically reduces the effects of
parasitic coupling, that eliminates spatial and phase aberrations
imparted onto the optical read-out beam as a result of workpiece
surface roughness, and that is capable of operating in on-line or
off-line modes.
More specifically, a distributed source or, equivalently, an array
of acoustic waves is generated in a first workpiece that has been
or is being welded to a second workpiece. The acoustic waves arrive
at the weld site in phase with respect to each other and, if the
weld has not yet formed, are reflected from the interface between
the two workpieces and illuminate the outer surface of the first
workpiece. As the weld forms, an increasing portion of the acoustic
waves is transmitted to the second workpiece, where they illuminate
its outer surface. The acoustic waves that illuminate the outer
surface of either of the workpieces (after having been transmitted
through or reflected from the weld site) arrive substantially in
phase with respect to each other over a locus at the outer surface
and vibrate the outer surface.
The weld information acquired by the acoustic waves is read-out
with a second optical beam that is reflected from the locus
locations at the outer surface and is phase modulated by its
vibrations. The phase modulated read-out beam is then coherently
combined with a reference beam, and the resulting beat frequency is
used to extract information about the weld, using either heterodyne
or homodyne detection techniques.
Reading out only in-phase portions of the acoustic waves provides
acoustic-wave directivity, and thereby reduces the effects of
parasitic coupling. Acoustic waves that arrive at the outer surface
from locations other than the weld site generate substantially
out-of-phase vibrations over the outer surface and cancel out. If
the acoustic-wave excitation and read-out patterns are suitably
scanned, then the present invention functions as an acoustic
imaging system. In general, spurious acoustic signals, including
reflections and acoustic scattering from random imperfections
within the workpiece or on its surface, can interfere with the
detection process. In the present invention, the phased-array
excitation and detection scheme significantly reduces signals from
spurious noise sources, while increasing the level of the acoustic
signal from the desired spatial location within, or on the surface
of the workpiece.
In the preferred embodiment, a narrow annular ring-shaped pulsed
optical transmitter source beam ("ping" beam) is used to generate
the array of acoustic waves in the first workpiece, through either
thermoelastic or ablation effects, and a narrow annular ring-shaped
beam with a different diameter than the transmitter beam is used
for the optical read-out beam.
Although the main function of the transmitter and receiver arrays
is to provide enhanced spatial resolution, they provide secondary
benefits as well. When compared to single spot laser acoustic wave
generation, the array approach greatly enhances the effective
available strength of the acoustic wave. One of the reasons for
this enhancement arises is that the acoustic wave may be
concentrated by focusing it into the area where it is needed. In
addition, the laser power density may be restricted to remain in
the thermoelastic regime to avoid cosmetic damage to the workpiece
surface. Since the arrays permit relatively large illumination
areas, the strength of the acoustic wave at the focal plane is
proportionately enhanced and is mainly limited by the available
laser pulse energy, rather than surface damage.
Another advantage is also realized with the array approach. If an
ablative mode of acoustic wave formation is employed, then a plasma
is formed by the ping laser. If the ping laser and read-out laser
are incident on the same workpiece surface (single-sided
operation), the ping beam and reflected read-out beam may be
partially absorbed by the plasma, unless the receiver site is
sufficiently spaced from the pinging site. The annular ring arrays
permit the ping and read-out beams to be coaxial with different
diameters. This avoids the absorption problem because there is no
spatial overlap between ping and read-out beams.
The annular ring geometry also improves the quality of the acoustic
waves that are generated. Many types of welders cause a substantial
and variable deformation of the workpiece surface (the weld
"bead"). Laser ultrasonic sources that form the acoustic waves
directly below the weld bead are aberrated by the surface
deformation, causing a corresponding aberration in the generated
acoustic waves. With an annular ring geometry, the ring can be made
larger than the deformed surface portion to avoid aberration of the
ping beam, and the corresponding aberration of the acoustic
waves.
The invention also compensates for phase aberrations and spatial
intensity variations induced in the optical read-out beam by the
optically rough vibrating surface by providing several wavefront
compensation options.
The preferred option for most applications is a double-pumped
phase-conjugate wavefront compensation system ("optical scrubber").
The phase-conjugate optical scrubber utilizes a double-pumped phase
conjugate mirror that removes the phase and spatial intensity
aberrations from the distorted read-out beam to yield a surrogate
read-out beam with a clean, planar wavefront. As a result, the
entire surrogate read-out beam can be coherently combined with a
reference beam to dramatically improve the signal-to-noise ratio
over prior laser-ultrasonic systems.
The phase-conjugate optical scrubber not only compensates for
spatial phase aberrations imposed on the read-out beam by surface
irregularities, it also compensates for spatial intensity
aberrations imposed on the read-out beam by surface anomalies (rust
spots, digs, etc.). Furthermore, the scrubber coherently sums the
phase modulations induced by the surface vibrations, which is a
critical requirement for coherent acoustic phased-array receiver
beam formation. In addition, the phase-conjugate scrubber can
"track out" whole-body motion of the workpiece, as well as
differential low frequency vibrations of the workpiece (frequencies
that fall within the temporal bandwidth of the optical scrubber),
while preserving the desired high frequency ultrasonic response.
Whole body noise sources and vibrations that are beyond the
bandwidth of the scrubber can be tracked out with conventional
electronic post-processing, while still preserving the desired
ultrasonic signal.
A second scrubber system is an innovative adaptive optical
wavefront scrubber using spatial light modulators (SLMs), such as
membrane light modulators (MLMs) or liquid crystal light valves
(LCLVs), configured into a closed-loop architecture. The aberrated
read-out beam is reflected from the output port of the SLM, which
contains a servo-controlled reflectors that planarizes the read-out
beam's wavefront. A feedback loop is provided by directing a small
fraction of the read-out beam, after reflection from the output
port, to the input port of the SLM, where it is optically
interfered with a plane-wave reference beam. The feedback loop
enables the SLM to configure its 2-dimensional output port phase
map so that phase distortions in the read-out beam are corrected,
while global, high frequency ultrasound signals are allowed to pass
through undisturbed.
A third scrubber system is similar to the second system described
above, except that traditional adaptive optical 2-dimensional phase
modulators (such as deformable mirrors or PZT arrays) are used in
conjunction with conventional wavefront error sensors and computer
algorithms that dynamically update the 2-dimensional spatial phase
modulator.
All the scrubber systems that utilize SLMs are capable of tracking
rapidly changing environmental distortions (such as vibrations, air
turbulence, etc.), making them particularly suitable for on-line
applications, such as inspection of moving parts. In addition, SLMs
typically have higher photosensitivities than DPCMs. For example,
the SLM can respond at its specified frame rate (typically in the
range of 10 milliseconds to 100 microseconds) for an input optical
intensity on the order 0.1 milliwatt per square centimeter. With
image intensifiers, this sensitivity can be improved so that input
intensities in the range of microwatts to nanowatts per square
centimeter will suffice. Such high SLM optical sensitivities allow
the present ultrasonic inspection system to be used for evaluation
of workpieces with very low surface reflectivities and/or with
extremely low optical damage thresholds.
These and other features and advantages of the invention will be
apparent to those skilled in the art from the following detailed
description of preferred embodiments, taken together with the
accompanying drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a, described above, is sectional view of a prior weld
inspection technique in which a welded workpiece is being probed by
an ultrasonic beam.
FIG. 1b is a graph corresponding to FIG. 1a, of an inspection
output signal as a function of time.
FIG. 1c is a sectional view of a welded workpiece illustrating the
effect of a parasitic acoustic coupling source on the ultrasonic
probe beam of FIG. 1a.
FIG. 1d is a graph corresponding to FIG. 1c, of an inspection
output signal as a function of time.
FIG. 2a is a block diagram of a single-sided laser-ultrasonic weld
inspection system constructed in accordance with the present
invention.
FIG. 2b is a block diagram of a double-sided laser-ultrasonic weld
inspection system constructed in accordance with the present
invention.
FIG. 3 is a cross-sectional schematic diagram of a laser-ultrasonic
weld inspection system for off-line operation.
FIG. 4a a schematic diagram of a wavefront compensated
interferometer embodiment utilizing a double-pumped phase
conjugator (DPCM).
FIG. 4b is a schematic diagram of a DPCM wavefront compensated
interferometer embodiment utilizing a self-aligning PCM.
FIG. 4c is a schematic diagram of a wavefront compensated
interferometer embodiment utilizing a self-aligning triple-pumped
phase-conjugate mirror (TPCM).
FIG. 4d a schematic diagram of a wavefront compensated
interferometer embodiment utilizing a DPCM and a double-pass Bragg
cell configuration.
FIG. 5a is a schematic diagram of a wavefront compensated
interferometer embodiment utilizing a membrane-based spatial light
modulator (MLM).
FIG. 5b a schematic diagram of a wavefront compensated
interferometer embodiment utilizing "traditional" adaptive
optics.
FIG. 6 is a sectional view of a welded workpiece illustrating the
power distribution of a circular acoustic array at the weld
interface, taken along the section line 6--6 of FIG. 3.
FIG. 7a is an enlarged sectional view of the welded workpieces of
FIG. 3 illustrating the acoustic amplitude distribution of a
circular acoustic array at the weld interface.
FIG. 7b is an enlarged sectional view of the welded workpieces of
FIG.3 illustrating the sensitivity curve of the read-out light ring
for acoustic waves that originate at the weld interface.
FIG. 7c is a enlarged sectional view of the welded workpieces of
FIG.3 illustrating the composite transmitter-to-read-out light ring
coupling curve for a transmitter light ring diameter that is twice
that of the read-out light ring diameter.
FIG. 8 is a block diagram of a laser-ultrasonic weld inspection
system for on-line operation.
FIGS. 9a and 9b are elevation views taken along the section line
6--6 of FIG. 3, illustrating different light ring geometries that
may be utilized.
DETAILED DESCRIPTION OF THE INVENTION
FIGS. 2a and 2b illustrate the general principles of the invention.
As illustrated in FIG. 2a, it generally consists of a laser
acoustic source beam ("pinger" beam) 2 that is focused by a lens 3
to form a spatial light pattern 7 on the surface 4 of a workpiece 6
for generating an array of acoustic waves (an acoustic probe beam)
8 in the workpiece 6, a read-out beam 10 that is focused by a lens
5 to form a second spatial light pattern 9 on the workpiece surface
4 for detecting the acoustic probe beam 8 after it have probed an
inspection site 12 in the workpiece 6, such as a weld or flaw 12,
and signal processor 19 for extracting information about the weld
or flaw 2 from the phase modulated reflected read-out beam 16. In
the preferred embodiment, a read-out beam wavefront compensator 14
is used to compensate for the phase and amplitude aberrations
imparted onto the reflected read-out beam 16 by the workpiece
surface 4. The reflected read-out beam 6 is directed to wavefront
compensator 14 and signal processor 19 by beam director 18,
preferably a beam splitter.
FIG. 2a illustrates a configuration in which the pinger beam 2 and
read-out beam 10 are incident on the same workpiece surface 4. FIG.
2b illustrates a configuration in which the pinger and read-out
beams are incident on different workpiece surfaces, preferably a
workpiece surface 17 that is opposite the surface 4 that the pinger
beam 10 is focused on.
In addition to the two beam configurations illustrated in FIGS. 2a
and 2b, there are several types of read-out beam compensators 14
that may be used, as will be discussed below. The best combination
of beam configurations and read-out beam compensators is dependent
on the particular application.
For small area pinger beam spatial light patterns 7 it is
preferable to offset the light pattern 7 from the area being probed
12 so as to avoid damaged surface areas 11 (such as areas damaged
by the welding beam). However, as a consequence of the distributed
nature of the spatial light pattern 7, large offsets require that
the acoustic beam 8 propagate at large angles to the workpiece
surface 4 normal. Large propagation angles can degrade temporal
resolution unless the spatial light pattern 7 width, projected in a
direction parallel to the propagation of the acoustic probe beam 8,
is much smaller than 1 mm. By lens 3 a cylindrical lens, one can
minimize this spatial light pattern width and avoid the problem of
air breakdown that can result from focusing to small spot sizes
with conventional lenses. In some cases, it is also preferable to
offset the receiver spatial light pattern 9, to further
discriminate against spurious acoustic signals. However, to avoid
degradation in temporal resolution, the same spatial light pattern
width restrictions apply. Specific preferred spatial light pattern
shapes will be discussed in more detail below.
A preferred embodiment for off-line inspection of relatively small
size workpieces is illustrated in FIG. 3. For illustration
purposes, the invention will be described in the context of a weld
inspection application, wherein a "keystone" weld bridge 33 is
located between two metal workpieces 27 and 31. It is assumed that
the quality of the weld 33 is ascertainable by measuring its
diameter.
A pulsed transmitter laser 44 generates a pulsed beam 46 that is
focused by a lens 48 and axicon 50 to form a spatial light pattern
52, such as a "ring" pattern, on the surface 25 of one of the
welded workpieces 27. The spatial light pattern 52 generates an
array of acoustic waves 40 through either thermoelastic effects or
through ablation of material from the surface.
For operation in the thermoelastic regime the power density of the
pulsed beam 46 at the surface 25 should be kept below approximately
10 MW/cm.sup.2. At such power densities, the acoustic waves 40 are
generated by the thermoelastic stresses and strains that are
induced by localized heating of the material. The main advantage of
operating in the thermoelastic regime is that the material is not
damaged by the pulsed beam 46.
It is also possible to operate in the ablation regime by keeping
the power density of the pulsed beam 46 above 10 MW cm.sup.2. In
the ablation regime, material is ablated from the surface 25,
producing a net reaction force against the surface 25. This force
produces a stress at the surface which in turn generates
compression-mode acoustic waves 40. The advantage of operating in
this power regime is that the acoustic waves 40 produced via
ablation are stronger than those produced via thermoelastic
effects. The main disadvantage is that the material is damaged at
or near the surface 25 through the ablation process.
The acoustic waves 40 propagate through the first workpiece 27,
arrive in phase at the weld 33 and are coupled into the second
workpiece 31 through the weld 33. The acoustic waves 40 then
propagate through the second workpiece 31 and vibrate its outer
surface 35. The acoustic waves 40 arrive substantially in phase
over an annular ring locus at the outer surface 35.
The vibrating surface is then probed interferometrically. This is
accomplished by using a second laser 54, preferably a
continuous-wave (CW) or long-pulse laser, to generate a second
optical beam 56 which is subsequently divided into a diffracted
reference beam 58 and an undiffracted read-out beam 60 by a Bragg
cell 62 that is driven by an rf power source 63. The diffracted
reference beam 58 is shifted in frequency by Bragg cell 62 and
directed to a combination wavefront compensator/interferometer 72
(wavefront compensated interferometer) by mirror 73. The
undiffracted read-out beam 60 is used to read-out the acoustically
induced displacement of surface 35.
The intensity of read-out beam 60 is preferably much higher than
the intensity of diffracted reference beam 58. This way, the rf
power required to drive Bragg cell 62 is relatively low, which
reduces the potential for rf electrical interference in the
processing electronics 86. A high intensity read-out beam 60 is
also preferable in order to help overcome the low optical
reflectivity and/or high optical scattering that are typically
exhibited by workpieces.
The read-out beam 60 passes through a beam splitter, 61 that splits
a portion of the read-out beam into an optional compensator input
beam 59. The use of the optional input beam 59 will depend on the
type of wavefront compensator used. For DPCM and conventional
adaptive optics compensators, beam 59 is not used. For light valve
compensators, which typically have high sensitivity, only a few
microwatts of power is required in beam 59. The function of beam 59
will be explained in more detail below.
The read-out beam 60 then passes through a polarizing beamsplitter
63, that is oriented to transmit the read-out beam 60, and through
a quarter-wave plate 65, which converts the read-out beam's linear
polarization to circular polarization. The read-out beam 60 is then
focused by a lens 66 and axicon 68 to form a spatial light pattern
64, preferably a ring-shaped pattern, on the vibrating surface 35
over the locus. The geometry and function of the transmitter ring
52 and read-out ring 64 will be discussed below.
A portion of the read-out beam 60 is reflected and is
phase-modulated by the vibrating surface 35. The modulated read-out
beam 70 is then collected by the lens 66 and axicon 68 and directed
back through quarter-wave plate 65, where its circular polarization
is converted back a linear polarization that is orthogonal to its
original linear polarization. The linearly polarized modulated
read-out beam 70 is reflected by polarizing beamsplitter 63 and
directed to wavefront compensated interferometer 72.
The preferred intensity ratio between the read-out beam 60 and the
reference beam 58 is also dependent on the type of wavefront
compensator that is used in interferometer 72. With a DPCM
wavefront compensator, the intensity ratio is preferably chosen so
that the intensity of reference beam 58 and modulated read-out beam
70 are approximately equal. This enhances the response speed of the
DPCM.
The wavefront compensated interferometer 72 removes any spatial
phase aberrations induced on the modulated read-out beam 70 as a
result of its reflection from the optically rough vibrating surface
35, coherently combines it with reference beam 58 and generates an
electronic beat frequency signal. The beat frequency signal 84 is
sent to a processor 86 which analyzes the beat frequency to extract
information about the weld 33.
For moderate speed applications, the preferred wavefront
compensator for use in the wavefront compensated interferometer 72
is the double or triple-pumped phase conjugate mirror. Preferred
double-pumped phase conjugate mirror (DPCM) embodiments are
illustrated in FIGS. 4a-4d. For some examples of general laser beam
cleanup using double-pumped phase conjugate mirrors, see U.S. Pat.
No. 4,991,177, entitled "LASER BEAN CLEAN-UP USING MUTUALLY PUMPED
PHASE CONJUGATION" , issued Feb. 5, 1991 to T. Y. Chang, et al.,
and U.S. Pat. No. 4,911,537, entitled "BIRD-WING PHASE CONJUGATION
USING MUTUALLY INCOHERENT LASER BEAMS", issued Mar. 27, 1990 to
M.D. Ewbank.
The primary function of the DPCM is to remove spatial and intensity
aberrations induced on the modulated read-out beam 70 as a result
of its reflection from the optically rough vibrating surface 35. A
DPCM with a single commercially available photorefractive crystal
148 is more cost effective than the light valve embodiments
described below. Furthermore, since thick volume holograms are
written in the photorefractive crystal 148, the diffraction
efficiency is much higher than the relatively thin holograms
written in ferroelectric light valves. Another advantage of the
DPCM embodiment over light valve embodiments is that some of the
DPCM configurations permit auto alignment of the reference beam 58
with the modulated and compensated read-out beam 176. Thus, the two
beams exactly overlap at detectors D.sub.1 and D.sub.2.
For many applications, the preferred embodiment of a wavefront
compensated interferometer 72 using a DPCM is illustrated in FIG.
4a. Reference beam 58 is split by beamsplitter 77 into a
compensator reference beam 79 and a detector reference beam 81. The
compensator reference beam 79 passes through a polarizing
beamsplitter 83 that is oriented to transmit the beam's linear
polarization, and through a Faraday rotator 75, which rotates its
linear polarization by 45 degrees. The compensator reference beam
79 is then focused into the +c-face 85 of a photorefractive crystal
148 by lens 87. The photorefractive crystal is preferably
BaTiO.sub.3, but it could also be Ba.sub.2-x Sr.sub.x K.sub.1-y
Na.sub.y Nb.sub.5 O.sub.15, KNbO.sub.3, Sr.sub.1-x Ba.sub.x
Nb.sub.2 O.sub.6 or any other photorefractive crystal. In addition,
the double-pumped phase conjugator implementation is not limited to
a photorefractive crystal. Other types of nonlinear materials could
be used in place of the photorefractive crystal.
The modulated read-out beam 70 is focused into an a-face 89 of the
crystal 148 by lens 91, where it combines with the compensator
reference beam 79 to write mutual photorefractive index gratings
(not shown). The modulated read-out beam 70 diffracts off the
mutual gratings and emerges as a clean (scrubbed) compensated
read-out beam Compensated read-out beam 71 is actually the
phase-conjugate of the compensator reference beam 79, so it has the
same frequency, wavefront and polarization as the compensator
reference beam 79 The compensated read-out beam 71 passes through
Faraday rotator 75, which rotates its polarization by another 45
degrees. Since the compensated read-out beam's polarization is now
orthogonal to the polarization of the compensator reference beam
79, it is reflected by polarizing beamsplitter 83 towards
interferometric beamsplitter 93, preferably a 50/50 beamsplitter.
The portion of beam 71 that is transmitted through beamsplitter 93
is combined, in angular registration, with the portion of beam 81
that is reflected by beamsplitter 93. The combined beams 95 are
focused into detector D.sub.1 by lens 97. Detector D.sub.1 detects
the beat frequency between the combined beams 95 and produce an
output electronic signal with a spectrum that is centered on an
intermediate output frequency f.sub.1.
Similarly, the portion of compensated read-out beam 71 that is
reflected by beamsplitter 93 is combined, in angular registration,
with the portion of beam 81 that is transmitted by beamsplitter 93.
These combined beams 97 are focused into detector D.sub.2 by lens
99. Detector D.sub.2 detects the beat frequency between the
combined beams 95 and produce an output electronic signal with a
spectrum that is centered on an intermediate output frequency
f.sub.1. The phase modulations on the portions of beam 71 that are
transmitted and reflected by beamsplitter 93 are 180 degrees out of
phase with respect as a consequence of reflection and transmission
from the beamsplitter. However, amplitude variations (possibly due
to laser instability) are not phase shifted by beamsplitter 93.
D.sub.1 and D.sub.2 's output signals are sent to a differential
amplifier 101, which performs a differencing operation on the two
detector output signals. Amplitude fluctuations that are present in
both detectors' output signals are not 180 degrees out of phase,
and therefore cancel. Since the phase modulations (from the signal
of interest) in detector output are 180 degrees out of phase with
respect to each other, they are doubled by the differential
amplifier 101. The output 84 of the amplifier 101 is sent to
processor 86 (shown in FIG. 3) for extracting information about the
weld from the beat frequency. Existing electronic post-processing
techniques (phase-locked loops, tracking circuits, etc.) can be
used to compensate for acoustic vibrations, which are typical in a
manufacturing environment.
One limitation with the wavefront compensated interferometer
embodiment of FIG. 4a is that it is not self-aligning. The angles
and positions of most elements must be carefully adjusted and
controlled. A self-aligning DPCM wavefront compensated
interferometer is illustrated in the embodiment of FIG. 4b. Mirror
151 directs reference beam 58 to a frequency shifter, preferably a
Bragg cell 150 that is driven by an AC voltage source 152, that
imparts a frequency shift of between 20 and 40 MHz on beam 58. This
allows the signal monitor 153 to operate in a heterodyne mode.
Reference beam 58 is then passed through a variable half-wave plate
154 and is directed to a first polarizing beamsplitter 156. The
polarizing beamsplitter 156 splits the reference beam into a
compensator reference beam 158 and a detector reference beam 160 by
reflecting the vertical polarization component of the reference
beam 58 and transmitting its horizontal polarization component. The
portions of the original reference beam 58 that go into the
compensator reference beam 158 and the detector reference beam 160
can be controlled by varying the angle of the half-wave plate 154.
For example, if the half-wave plate 154 is adjusted so that the
reference beam 58 has a 45 degree linear polarization, then half
the power in the reference beam 58 will go to the compensator
reference beam 158 and half will go to the detector reference beam
160 (assuming no absorption losses).
The detector reference beam 160 passes through a Faraday rotator
162, which rotates its polarization by 45 degrees. The beam is then
focused by a lens 164 into a second phase-conjugator 166,
preferably a self-pumped phase conjugator implemented with a second
photorefractive crystal. The phase-conjugate mirror 166 generates a
phase-conjugate 168 of the detector reference beam 160 which
counter-propagates back through the Faraday rotator 162 where its
polarization is rotated by 45 degrees (making the polarization of
the phase-conjugate detector reference beam 168 vertical), and back
to polarizing beamsplitter 156, which reflects beam 168.
The compensator reference beam 158 passes through a second Faraday
rotator 170, which rotates its vertical polarization by 45 degrees.
It then passes through a second half-wave plate 172, which rotates
its polarization by another 45 degrees (making the polarization
horizontal), and is focused by a lens 174, preferably a cylindrical
lens, into the +c-face 85 of photorefractive crystal 148.
The modulated read-out beam 70 is focused into an a-face 89 of
crystal 148 by lens 146. Beams 70 and 158 write mutual
photorefractive index gratings (not shown) in the photorefractive
crystal 148. The temporally modulated readout beam 70 diffracts off
the mutual gratings to form the phase-conjugate of the compensator
reference beam 176, and the compensator reference beam 158
diffracts off the mutual gratings to form the phase-conjugate of
the modulated read-out beam (not shown). This cross-readout process
results in the temporal phase modulations of the modulated read-out
beam 70 (induced by reflection from the vibrating surface) being
coupled over to the phase-conjugate compensator reference beam 176
without a corresponding coupling of the spatial phase aberrations
(or intensity drop-outs) imparted on the modulated read-out beam 70
by reflection from the optically rough or imperfect surface of the
welded workpiece (not shown). The result is a phase-conjugate
compensator reference beam 176 that has both the desired temporal
modulation of the modulated read-out beam 70 and the clean
wavefront of the compensator reference beam 158.
The phase-conjugate compensator reference beam 176
counter-propagates with respect to the compensator reference beam
158. It passes back through the lens 174 and through the half-wave
plate 172, which rotates its horizontal polarization by -45
degrees. The polarization rotation is -45 degrees because the
phase-conjugate compensator reference beam 176 is a "time-reversed"
replica of the compensator reference beam 158 and the half-wave
plate 172 is a reciprocal optical element. The Faraday rotator 170
rotates the polarization of the phase-conjugate compensator
reference beam 176 by +45 degrees so that the polarization is now
horizontal again (the Faraday rotator 170 is a non-reciprocal
optical element). This allows the beam to pass through the
polarizing beam splitter 156. The combined phase-conjugate
compensator reference beam 176 and phase-conjugate detector
reference beam 168 are then directed to the signal monitor 153 for
detection of the beat-frequency between them.
Beams 176 and 168, after passing through and reflecting off
(respectively) polarizing beamsplitter 156 are orthogonally
polarized. In order to coherently combine these beams for
heterodyne detection, the pair of beams are directed to a second
polarizing beamsplitter 190 that is rotated 45 degrees with respect
to polarizing beamsplitter 156. Beamsplitter 190 functions as an
interferometric beam combiner, so that the pair of beams, 191 and
192, that emerge from beamsplitter 190 each contain coherent
contributions of the original input beams 176 and 168. By
conservation of energy, beams 191 and 192 each contain the desired
phase-difference information of input beams 176 and 168, but are
180 degrees out of phase with respect to each other. Beams 191 and
192 are then directed into separate optical detectors D.sub.1 and
D.sub.2 by mirror 193 and lenses 194 and 197. The electrical
outputs of detectors D.sub.1 and D.sub.2 are sent to differential
amplifier 101 that performs the same differencing function
described above in connection with FIG. 4a. The differential
amplifier output 84 is directed to processor 86 (not shown) for
demodulation. Signal monitor 153 functions as a balanced detection
circuit, with common-mode rejection of additive noise, but with
twice the signal level for the desired output signal information,
due to the 180 degree phase shift between beams 191 and 192,
coupled with the differencing function of differential amplifier
101.
FIG. 4c illustrates an alternative wavefront compensated
interferometer embodiment that is also self-aligning, and that
employs a triple-pumped phase-conjugate mirror (TPCM). The
embodiment of FIG. 4c is a variation on the embodiment of FIG. 4b,
and common elements are labeled with the same element numbers. The
main differences between the embodiments of FIGS. 4b and 4c is the
use of an additional pump beam 58a derived from reference beam 58,
and the use of a different type of signal monitor 153. Pump beam
58a is split out of reference beam 58 by beamsplitter 171 and
directed to the a-face of the crystal 148 by mirror 173 and lens
175. Beam 176 is the phase conjugate of compensator reference beam
158 and consists of two co-aligned beams at the input frequencies
of modulated read-out beam 70 and reference beam 58. If the
compensator reference beam 158 is unaberrated, then the phase
conjugate beam 176 is also. However, even if small aberrations are
present on beam 158, they will appear in both of the two co-aligned
beams (that have the frequencies of beams 70 and 58) that make up
the phase conjugate beam 176. Since the aberrations appear in both
of the co-aligned beams, their wavefronts will be matched and high
detection efficiency will result. Note that the TPCM, like the
DPCM, works most efficiently if no one of the three input beams
58a, 70 and 158 form internal gratings via cross beats between any
other input beam. For this reason, Bragg cell 150 is used to offset
beam 158 in frequency from beams 58a and 70. The frequency
difference between any pair of input beams should preferably exceed
the inverse of the grating formation time.
One drawback to the embodiment of FIG. 4c is that signal monitor
153 is not a balanced detector like the ones used in the
embodiments of FIGS. 4a and 4b. However, laser amplitude noise can
still be canceled. A beamsplitter 177 is used to split reference
beam 58 into reference beam portion 58b and compensator reference
beam 158. Lens 154 focuses reference beam portion 58b into detector
D.sub.1. The two co-aligned beams (that have the frequencies of
beams 70 and 58) that make up beam 176 are focused into detector
D.sub.2 by lens 197. In this scheme, detector D.sub.1 is a direct
detector and detector D.sub.2 is a heterodyne detector. Laser
amplitude noise is equally present on both detector signals and, if
the input optical signals are equal in magnitude, subtracting the
detector outputs with differential amplifier 101 cancels the laser
amplitude noise. An automatic gain control (AGC) circuit is used at
the output of detector D.sub.1 to balance the magnitude of the
detector signals.
An advantage of the embodiment of FIG. 4c, results from the
temporal response properties of the TPCM. The temporal response to
a perturbation of an existing grating system in the crystal 148,
which results in a partial set of new gratings being formed, is
much faster than the time to build up the gratings from a dark
state. Therefore, the reference beam 58a input can maintain a
grating system during read-out beam 70 intensity drop-outs, which
speeds up the acquisition of the correct gratings when the read-out
beam 70 re-appears, subject to incoherent erasure of the gratings
by the various input beams.
In applications which require very high temporal resolution, it is
advantageous to use correspondingly higher offsets between read-out
and reference beam frequencies to produce higher intermediate
frequencies out of the heterodyne detectors. A preferred method of
achieving these frequency offsets is to employ a double-pass Bragg
cell configuration, as illustrated in the wavefront compensated
interferometer embodiment of FIG. 4d. Compensator reference beam
158 is split into two equal intensity beams 158a and 158b by Bragg
cell 150. The undiffracted component 158a is the reference beam of
the DPCM, whose phase-conjugate reflection 176 carries the
modulated read-out beam's 70 phase modulation (as in the
embodiments of FIGS. 4b and 4c). The diffracted portion 158b is
frequency upshifted by the Bragg cell driving frequency f.sub.2. It
is reflected by mirror 166, which is preferably a phase-conjugate
mirror into which beam 158b is focused by lens 164. However, mirror
166 may also be a corner reflector or a "cat's eye" retroreflector.
The reflected beam 159 returns to Bragg cell 150, where it is again
split into a diffracted beam 159' and undiffracted beam 159". The
undiffracted beam 159" (which retains the single frequency upshift
f.sub.2) is reflected by mirrors M2 and M3 and is focused onto
detector D.sub.2 by lens 197. The diffracted beam 159' is again
upshifted in frequency (for a total frequency shift of 2f.sub.2)
and, after polarization rotation, is focused onto detector D.sub.1
by lens 194.
Similarly, beam 176 is split into a diffracted beam 176'
(downshifted in frequency by f.sub.2) and an undiffracted beam
176". The diffracted beam 176' is parallel to beam 159" (which has
a frequency upshift of f.sub.2) and is also focused onto detector
D.sub.2 by lens 197. Thus,the beat frequency out of detector
D.sub.2 is equal to 2f.sub.2. The undiffracted beam 176" is
parallel to diffracted beam 159' (which has a frequency upshift of
2f.sub.2) and is also focused onto detector D.sub.1. Thus, the beat
frequency out of detector D.sub.1 is also equal to 2f.sub.2.
A secondary advantage of the embodiment of FIG. 4d is that the
Bragg cell driver 152 at frequency f.sub.2 cannot effectively
couple electrical noise into the outputs of detectors D.sub.1 and
D.sub.2. Although the common-path error cancellation feature found
in the embodiment of FIG. 4c is not available in this embodiment,
the system is substantially self-aligning. Furthermore, laser
amplitude noise is effectively canceled by the same balanced signal
monitor 153 found in the embodiment of FIG. 4a.
The beat frequency output 84 of the differential amplifier 101 in
all of the wavefront compensated interferometer embodiments
discussed above are amplified and phase detected by processor 86 to
yield an electronic pulse whose output shape and intensity yields
the desired measure of the weld size and quality.
Although the DPCM and TPCM-based wavefront compensated
interferometers of FIGS. 4a-4d are generally preferred for off-line
applications, on-line in-situ applications often require higher
speed operation than can be provided by existing DPCMs and TPCMs.
Alternative wavefront compensators are available which are capable
of sub-millisecond response speeds and higher sensitivities, as
will be described next. Many of the high-speed compensators that
can be employed in the wavefront compensated interferometer are
described in the context of adaptive-optics imaging. For the
present ultrasonic inspection system, different geometries that
those described for adaptive optics imaging are required, as is
described below.
One approach is to use refracting/reflecting (closed loop) adaptive
optics, as illustrated in the wavefront compensated interferometer
embodiments of FIGS. 5a and 5b. In FIG. 5a, high speed compensation
is achieved with a compensating deformable mirror realized as
membrane-based spatial light modulator 180 (MLM), which utilizes an
array 182 of discrete membrane micro-mirrors that are driven by a
microchannel plate current amplifier 208. A detailed description of
this wavefront compensation system can be found in Celestino J.
Gaeta et al., "Characteristics of Innovative Adaptive-Optics Servos
That Use Membrane-Based Spatial Light Modulators", J. Opt. Soc. Am.
A, vol. 11, No. 2, pages 880-894 (1994).
The temporally modulated read-out beam 70 is directed to the
read-out side 178 of a membrane-based spatial light modulator 180.
A lens array 184 focuses the modulated read-out beam 118 onto a
reflective membrane array 182, where it is retro-reflected. The
retro-reflected modulated read-out beam 185 is then partially
reflected by a beam splitter 186 and is directed by a mirror 188 to
a second beam splitter 190. The second beamsplitter transmits part
of the retro-reflected read-out beam 185 to beam splitter 218 and
reflects the rest towards a third beam splitter 192, which directs
the beam to the write side 194 of the spatial light modulator
180.
Reference beam 59 is expanded by lenses 187 and 189 and directed to
the write side 194 of the spatial light modulator 180 by mirrors
191 and 193, where it optically interferes with the retro-reflected
modulated read-out beam 185. This creates an interference pattern
202 on the photo-cathode surface 204 of the light modulator 180.
The modulated read-out beam 70 that reflects from the membrane
array 182 on the read-out side of the light valve 178 experiences a
phase shift that corresponds to the interference pattern on the
write side 194 of the light modulator 180. Since a portion of the
retro-reflected, temporally-modulated and phase-corrected read-out
beam is directed to the write side 194 to interfere with reference
beam 59, a feedback loop exists. After multiple iterations, the
retro-reflected, temporally-modulated read-out beam 185 is
substantially free from spatial-phase aberrations (wavefront
compensated).
Reference beam 58 is directed to a beamsplitter 218, where it is
where it combines with beam 185. The combined beams are directed to
lenses 194 and 197 (with mirror 195) of signal monitor 153 for
detection of the resulting beat-frequency. Although the MLM cannot
compensate as much of the speckled field (arising from scattering
off rough workpiece surfaces) as a phase conjugator, it need not
accommodate a large number of compensation sites in order to be
effective. Furthermore, it offers an optical efficiency advantage
over phase conjugators because the micro-mirror array 182
reflectivity is over 95%, compared to 40%-50% for DPCMs and TPCMs,
and 20% for real-time holograms.
In the wavefront compensated interferometer embodiment of FIG. 5b,
a "traditional"adaptive optics system is used for wavefront
compensation. The modulated read-out beam 70 is directed through a
first beam splitter 220 and to a deformable mirror 222, which
retro-reflects the beam 70. The retro-reflected read-out beam 224
is then directed by the first beamsplitter 220, a mirror 226, and a
second beam splitter 228 to a two-dimensional lens array 230. The
lens array 230 focuses the retro-reflected read-out beam 224 onto a
two-dimensional array of quadrant detectors 232 that sense the
wavefront error on that part of the beam. The signals from the
quadrant detectors 232 are sent to a processor 234 and are used to
adjust the voltage levels across piezoelectric actuators 236 in a
manner that causes the deformable mirror 222 deflection to minimize
the wavefront error in the retro-reflected read-out beam 224. The
portion of beam 234 that passes through beamsplitter 228 is
directed to a third beamsplitter 234 by mirror 225, where it
combines with reference beam 58. The combined beams are directed to
lens 197 and lens 194 (by mirror 227) of signal monitor 153, for
detection of the resulting beat-frequency.
An elevation view taken along line 6--6 of FIG. 3 is shown in FIG.
6, with transmitter light ring 52 and weld 33 shown as if you could
see through the workpieces for illustration. As explained above,
the transmitter beam light ring 52 has a different diameter than
the read-out beam light ring 64. The different ring diameters can
contribute to the enhancement of the signal-to-noise, including
suppression of spurious off-axis path noise sources, as will be
discussed below.
Referring back to FIG. 3, every point on the transmitter light ring
52 acts as a point source of acoustic waves 40 through either
thermoelastic or ablation effects. Therefore, the transmitter light
ring 52 actually generates an array of acoustic waves that arrive
in phase at points in the workpieces 27 and 31 that are centered
with respect to the transmitter light ring 52. At these points, the
acoustic waves 40 coherently "add", resulting in an acoustic "beam
"having a finite diameter that is smaller than would be produced by
a typical (1 mm to 2 mm) single laser illumination spot. Post
processing filtering is generally required to remove the lower
acoustic frequency components, thereby appreciably narrowing the
acoustic beam diameter. The diameter of the acoustic beam at any
given depth is a function of the transmitter light ring 52
diameter, the wavelength of the acoustic beam and the distance from
the transmitter light ring 52, and can in many cases be
approximated by the expression:
where .lambda. is the center wavelength of the acoustic band, and D
is the diameter of the light ring. This formula is valid in the
limit where the depth into the workpiece under inspection
approximates or exceeds D/2, which is typically the case.
Therefore, the diameter of the transmitter light ring 52 can be
adjusted so that the coherent addition of the acoustic waves 40 at
the weld 33 result in a narrow acoustic beam at the weld 33.
Furthermore, when generating shear waves, the transmitter light
ring 52 should have a diameter which produces a maximum shear wave
(i.e., focused) acoustic pattern at the desired internal probe
volume within the workpiece (the weld 33 in FIG. 3). Ideally, in
many weld applications, the acoustic waves 40 should combine so
that the acoustic beam they form is as wide as the maximum weld 33
diameter at the weld site. This large acoustic beam size applies to
the case depicted in FIG. 3, where the fusion size of the weld 33
can be ascertained in a single shot (or, more precisely, with the
receiver light ring at a fixed location on the workpiece 31 via
acoustic-wave energy transmission through the weld 33. In the case
where one desires to determine the dimensions a weld by scanning
across the welded region, then a smaller diameter acoustic beam
(relative to the weld width) would be preferable, so that a
detailed mapping of the weld 33 can be obtained. Either of these
conditions can be achieved by choosing appropriate focal lengths
for the lens 48 and axicon 50 for a given weld depth (the distance
between the surface 25 of the second workpiece 27 and the weld 33).
For example, if a weld with a maximum diameter of 1 mm is located 3
mm from the surface 25 of the second workpiece 27, then the
transmitter light ring 52 diameter should be approximately 8.5 mm
and the focal lengths of the lens 48 and axicon 50 should be chosen
appropriately.
FIG. 7a is an enlarged sectional view of the workpieces
illustrating this example. The transmitter light ring 52 is
approximately 8.5 mm in diameter and generates acoustic waves (not
shown) that coherently add at the weld 33. A cross-section of the
acoustic amplitude distribution 115 of the coherently combined
acoustic waves is shown. The acoustic beam diameter is defined as
the full-width-half-maximum of the central lobe 119a. In this
example, the 8.5 mm transmitter light ring 52 diameter results in
an acoustic beam diameter at the weld 33 that is approximately
equal to the weld 33 diameter (1 mm).
Ideally, the light ring thickness 109 should be infinitesimally
small so that all acoustic waves originating from the light ring 52
coherently add at the weld 33. As the light ring thickness 109 is
increased, some of the acoustic waves that originate at the light
ring 52 do not arrive in phase at the weld 33. Therefore, they
interfere destructively at the weld 33 and the amplitude of the
central lobe 119a decreases. Therefore, in the preferred
embodiment, the light ring thickness 109 should be less than 20
percent of the acoustic wavelength at the highest acoustic
frequency.
Referring back to FIG. 3, the transmitted acoustic waves 37 impinge
on the upper surface 35 of the second workpiece 31 and a portion of
these waves impinge on the read-out ring 64 area. Just as acoustic
waves generated by the transmitter light ring 52 arrive in phase at
the weld 33, all acoustic waves 37 that are transmitted through the
weld 33 and arrive at the read-out ring 64 arrive in-phase over a
locus underneath the read-out ring 64 because the weld 33 is
centered with respect to the read-out ring 64 This causes the
surface underneath the read-out ring 64 to vibrate in-phase.
Acoustic waves that are reflected from off-center interface sites
arrive substantially out-of-phase with respect to each other and
substantially cancel when averaged over the read-out ring 64 area.
The canceling out of parasitic reflections dramatically improves
the signal-to-noise ratio.
The same light ring thickness criteria that applies to the
transmitter light ring 64 applies to the read-out light ring 52. As
the read-out light ring thickness is increased, a larger portion of
the transmitted acoustic waves 37 destructively interfere at the
read-out ring 64, resulting in a smaller signal.
The read-out light ring 64 reflects from the vibrating surface 35
and is phase-modulated in accordance with the amplitudes at the
frequencies of the vibrations. As mentioned above, the acoustic
waves 40 produced by the transmitter ring 52 arrive in-phase at the
weld 33 and thus also arrive in-phase over a locus underneath the
read-out ring 64 after transmission through the weld 33.
The reflected, modulated read-out beam 70 is collected by the
axicon 68 and lens 66 and is directed back through the quarter-wave
plate 65, which converts its circular polarization to a horizontal
linear polarization and is reflected by polarizing beam splitter
63. The modulated read-out beam 70 and the reference beam 58 are
then directed to the wavefront compensated interferometer 72.
Referring back to FIG. 7a, another potential source of noise is the
existence of side-lobes 119b in the acoustic beam. Ideally, as
discussed above, the transmitter light ring diameter is chosen so
that the acoustic waves "add" to give a narrow central lobe 119a at
the weld 33. In practice, the amplitude profile of the acoustic
beam at the weld 33 contains side-lobes 119b that are not
completely suppressed by changing the diameter of the transmitter
light ring 52. These side-lobes 119b are a potential source of
noise because they extend beyond the boundaries of the weld
site.
One way to help mitigate this potential source acoustic-clutter
noise is illustrated in FIGS. 7b and 7c. FIG. 7b is a sectional
view of the welded workpieces with a cross section of the read-out
light ring sensitivity 64 as a function of position along the
interface. The read-out light ring sensitivity curve 121
illustrates how well acoustic waves originating from different
positions along the interface between workpieces 27 and 31 add up
at the read-out light ring 64. In this example, acoustic waves
arriving from the weld 33 site arrive substantially in phase and
therefore add up to give a strong acoustic signal (central lobe
127). The diameter of the read-out light ring 64 can be adjusted so
that the side-lobe structure 123 is substantially out of phase with
the side-lobe structure 119b of the transmitter light ring 52 of
FIG. 7a.
Since the transmitter-to-receiver coupling is defined by the
product of the transmitter power distribution at the weld and the
read-out light ring sensitivity curve, the signal contributions
from the side-lobes are suppressed. This is illustrated in FIG. 7c.
The side-lobe suppression is accomplished by making the read-out
light ring 64 diameter less than the transmitter light ring 52
diameter. The resulting transmitter-to-read-out light ring
composite coupling curve 125 has the desired narrow central lobe
127 with significant suppression of the side lobes 129.
In order to obtain good spatial resolution with annular sources and
receivers, it is required that the sound wave frequency content be
restricted to high frequencies, preferably starting at about 5 MHz.
A three-to-one, or four-to-one frequency range is typically
employed (for example 5 to 15 MHz or 5 to 20 MHz). This provides a
good compromise between temporal and spatial resolution.
For some applications, there are additional advantages to high
frequency sound wave sources. These include the ability to better
suppress undesired surface wave coupling (with single-sided
operation), and the ability to increase the system's sensitivity to
small flaws or grain size variation in metals.
There are a number of ways of achieving waveforms which emphasize
the desired high frequency content, and achieve good temporal
resolution. One approach is to use a short pulse burst from a short
pulse transmitter beam laser. Three pulses are a good compromise
for many applications. This generates a frequency band centered at
a frequency which is the inverse of the pulse spacing. However,
this leaves an undesired band centered on zero frequency as well.
Furthermore, this approach is optically challenging to implement
because of the high power levels required.
A preferred approach is to use a single short pulse laser
excitation which contains strong high frequency spectral
components, and extract the desired bands by filtering the
appropriate bands from the pulse out of the receiver. A preferred
way of filtering is to split the output pulse and multiply each
component by 0.5, invert them, and add them to the original pulse
with equal advance and delay relative to it.
A preferred embodiment for on-line use, to interrogate features
within a volume, is illustrated in FIG. 8. A pulsed transmitter
laser 44, preferably a YAG laser operating at a wavelength 1.06
microns, or a CO.sub.2 laser operating at a wavelength of 10.6
microns, is used to generate optical pulses 46 that are typically
between 2 and 20 ns in duration. The pulsed beam 46 is directed by
a dichroic beam splitter 88 (that only reflects light at the
transmitter laser 44 wavelength) to lenses 90 and 92 which expands
and collimates the beam 46.
A read-out laser 54, preferably a diode-pumped, continuous-wave (or
long-pulsed), doubled-YAG laser operating at a wavelength of 532
nm, is used to generate an optical beam 56 having both horizontal
and vertical linear polarization components. A polarizing beam
splitter 94 splits the optical beam 56 into a read-out beam 60 and
a reference beam 58 by reflecting the vertical polarization
component and transmitting the horizontal polarization
component.
A quarter-wave plate 96 changes the polarization of the read-out
beam 60 from vertical to circular. The read-out beam 60 then passes
through the dichroic beam splitter 88 and is expanded and
collimated by lenses 90 and 92.
The co-propagating read-out beam 60 and pulsed transmitter beam 46
are then reflected by an annular mirror 98. The transmitter beam 46
and read-out beam 60 are then focused by an annular lens 100 and
axicon 102 onto the surface 103 of a first workpiece 31 that is
being welded to a second workpiece 27. Both the transmitter beam 46
and the read-out beam 60 are formed into light patterns, preferably
ring-shaped patterns 108 and 110 respectively, on the surface of
the first workpiece 31 by the annular lens 100 and axicon 102. The
centers of the transmitter light ring 108 and the read-out light
ring 110 are aligned with the center of the weld 33 to be probed,
or are scanned across it if the workpieces are being inspected
while they move along an inspection line. Since the transmitter
beam 46 and read-out beam 60 have different wavelengths, the bulk
and/or grating dispersive property of the annular axicon 102 causes
the ring-shaped light patterns 108 and 110 to be of different
diameters for optimum side-lobe and clutter-noise suppression.
The annular mirror 98, lens 100, and axicon 102 allow a weld beam,
such as a laser or electron-beam welder beam 106, simultaneous
access to the workpieces 31 and 27, resulting in the ability to
acoustically probe the weld 33 while it is being formed (on-line).
The phased-array receiver can also be used in applications where
acoustic emission from the weld-joint generates an acoustic beam,
whose signature is to be sensed (which would eliminate the need for
the transmitter beam 46 that would otherwise be required to induce
acoustic waves in the workpiece). In a manner similar to the above
system, the acoustic emission can be sensed with a locus of
appropriately placed laser probe beams, thereby maximizing the
desired signal, while reducing background clutter noise and other
spurious acoustic signals.
In the embodiment of FIG. 8, the read-out light ring diameter is
made half that of the transmitter light ring diameter by choosing a
read-out beam 60 with a wavelength that is twice as long as the
transmitter beam 46 wavelength and using common dispersive optical
elements to form the readout and transmitter light rings on the
surface 103 of the first workpiece 31. It is preferable to use
transmitter and read-out light patterns that are not fully
circular, as illustrated in FIGS. 9a and 9b. The different ring
diameters help reduce cross-coupling from ring-to-ring, which could
be caused by surface coupling. For example, half rings could be
used for the transmitter light pattern 108 and/or the read-out
light pattern 110, as illustrated in FIG. 9a. Although, the
acoustic beam generated by a half-ring is asymmetric (narrow in one
dimension and wide in another dimension), with a loss in spatial
resolution along the wide part of the beam, there is a significant
advantage to this configuration. With single-sided operation, there
are additional coupling mechanisms from transmitter to receiver
which can restrict the ring geometry. Specifically, the transmitter
light ring 108 also generates surface or Raleigh waves which skim
the surface 103 of the workpiece 31 and arrive (in phase) at the
read-out light ring 110. Although the surface wave path length is
shorter than the path length for the double-pass compression waves
114 (which probe the weld), their velocity is almost half of the
compression wave 114 velocity. Thus, the surface waves can easily
arrive in temporal coincidence with the first arriving compression
wave 114 and obscure or otherwise interfere with it. With a rough
workpiece surface 103 the surface wave may be sufficiently
attenuated, over long propagation paths, that surface wave
interference becomes negligible. A semi-circular geometry maximizes
the surface wave path length, as discussed below, and thereby also
maximizes the surface wave attenuation.
If this is not the case, several alternative solutions are
available with short pulse operation. The simplest is to move the
transmitter and read-out rings 108 and 110 closer or further apart
to eliminate temporal coincidence. A more widely applicable
solution is to use semi-circular, oppositely configured, annular
rings rather than full circles for the transmitter and read-out
rings. This does not fully eliminate the surface wave coupling, but
does appreciably reduce it. One reason for the reduction in surface
wave coupling is that the effective propagation distance from a
source site to the various read-out points on the read-out ring 110
is greatly increased, thereby better attenuating the surface waves
through spreading and loss mechanisms.
There is another important effect that effectively eliminates short
path coupling with high frequency sound waves. A point source on
the transmitter ring 108 induces a surface wave which propagates to
the read-out ring 110 with unequal time delays and causes a
corresponding surface displacement. At any particular temporal
frequency, these displacements are sinusoidal and will induce a
large phase shift over the read-out semicircle 110. This phase
shift will be several multiples of 360 degrees, and the surface
wave induced surface displacements will be effectively integrated
over the read-out ring 110. Thus, the average surface displacement
induced by the surface wave will sum to substantially zero at the
high frequency end, and to low values at the low frequency end of
the pulse spectrum. The only exception results from a spherical
wave formed by the transmitter ring 108 semi-circle. Such a wave
effectively focuses at the ring center, expands, and arrives at the
read-out semi-circle ring 110 in phase. However, the propagation
distance is maximum for this type of coupling, specifically the sum
of the ring radii. Since the surface wave sound velocity is about
half that of the compression waves 40, it arrives appreciably later
in time than the compression waves 40 that probe the underlying
volume (for typical geometries). This differs from full ring
systems wherein the surface wave travel distance is the difference
in the ring radii. Usually, this geometry results in an overlap of
the surface wave and compression wave pulses. However, in rare
cases the semicircular geometry yields a pulse overlap and the full
ring designs can provide a solution to the problem. For such cases,
the surface wave pulse arrives ahead of the compression wave. The
Rayleigh arrival pulse amplitude can provide a useful calibration
check on the transmitter laser 46 intensity, if the attenuation
constant for this wave is known. For all these reasons, half-circle
transmitter and read-out light rings are preferred, for
single-sided operation.
In addition, the light patterns do not need to be continuous rings.
As shown in FIG. 9b, it is possible to use an array of discrete
light spots 107 for the transmitter light pattern 108 and/or the
read-out light pattern 110, as long as the light spots 107 are
arranged symmetrically about the weld 33. However, the strength of
the acoustic beam that is generated by the transmitter light
pattern 108 and the sensitivity of the read-out light pattern 110
may be reduced as the number of light spots 107 decreases because
this reduces the number of acoustic sources. Therefore, continuous
light rings are preferred, especially if the laser power must be
held below the surface damage threshold of the workpiece.
While several illustrative embodiments of the invention have been
shown and described, numerous variations and alternate embodiments
will occur to those skilled in the art. For example, a "ring-shape"
is not the only transmitter and read-out beam geometries that could
be used. Other geometries that will generate an acoustic array and
allow the read-out beam to detect only the acoustic waves that
probe the weld could be used. Such variations and alternate
embodiments are contemplated, and can be made without departing
from the spirit and scope of the appended claims.
* * * * *